crypto: Add tls-cipher-suites object

On the host OS, various aspects of TLS operation are configurable.
In particular it is possible for the sysadmin to control the TLS
cipher/protocol algorithms that applications are permitted to use.

* Any given crypto library has a built-in default priority list
  defined by the distro maintainer of the library package (or by
  upstream).

* The "crypto-policies" RPM (or equivalent host OS package)
  provides a config file such as "/etc/crypto-policies/config",
  where the sysadmin can set a high level (library-independent)
  policy.

  The "update-crypto-policies --set" command (or equivalent) is
  used to translate the global policy to individual library
  representations, producing files such as
  "/etc/crypto-policies/back-ends/*.config". The generated files,
  if present, are loaded by the various crypto libraries to
  override their own built-in defaults.

  For example, the GNUTLS library may read
  "/etc/crypto-policies/back-ends/gnutls.config".

* A management application (or the QEMU user) may overide the
  system-wide crypto-policies config via their own config, if
  they need to diverge from the former.

Thus the priority order is "QEMU user config" > "crypto-policies
system config" > "library built-in config".

Introduce the "tls-cipher-suites" object for exposing the ordered
list of permitted TLS cipher suites from the host side to the
guest firmware, via fw_cfg. The list is represented as an array
of bytes.

The priority at which the host-side policy is retrieved is given
by the "priority" property of the new object type. For example,
"priority=@SYSTEM" may be used to refer to
"/etc/crypto-policies/back-ends/gnutls.config" (given that QEMU
uses GNUTLS).

The firmware uses the IANA_TLS_CIPHER array for configuring
guest-side TLS, for example in UEFI HTTPS Boot.

[Description from Daniel P. Berrangé, edited by Laszlo Ersek.]

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20200623172726.21040-2-philmd@redhat.com>

1 files changed, 19 insertions, 0 deletions

diff --git a/qemu-options.hx b/qemu-options.hx
index 196f468786..ecc4658e1f 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -4567,6 +4567,25 @@ SRST
         string as described at
         https://gnutls.org/manual/html_node/Priority-Strings.html.
 
+    ``-object tls-cipher-suites,id=id,priority=priority``
+        Creates a TLS cipher suites object, which can be used to control
+        the TLS cipher/protocol algorithms that applications are permitted
+        to use.
+
+        The ``id`` parameter is a unique ID which frontends will use to
+        access the ordered list of permitted TLS cipher suites from the
+        host.
+
+        The ``priority`` parameter allows to override the global default
+        priority used by gnutls. This can be useful if the system
+        administrator needs to use a weaker set of crypto priorities for
+        QEMU without potentially forcing the weakness onto all
+        applications. Or conversely if one wants wants a stronger
+        default for QEMU than for all other applications, they can do
+        this through this parameter. Its format is a gnutls priority
+        string as described at
+        https://gnutls.org/manual/html_node/Priority-Strings.html.
+
     ``-object filter-buffer,id=id,netdev=netdevid,interval=t[,queue=all|rx|tx][,status=on|off][,position=head|tail|id=<id>][,insert=behind|before]``
         Interval t can't be 0, this filter batches the packet delivery:
         all packets arriving in a given interval on netdev netdevid are